For many years gas fired furnaces and appliances have used an ignition source referred to as a standing pilot. A standing pilot arrangement provides for a continuously burning flame adjacent the burner for the appliance. The standing pilot is usually monitored with a thermocouple or other heat sensing elements, and is very inexpensive and reliable in operation. With the advent of the rapid increase in the cost of fuels, attempts have been made to find other means for igniting burners in furnaces and appliances, such as water heaters. This search for an alternate ignition arrangement has been mandated in some localities by legislation which makes a standing pilot for ignition in new equipment a violation of law.
Two alternative ignition sources have been known for many years. The source which was most easily implemented was a source normally referred to as a spark ignition source. A spark ignition source is a spark gap across which a high potential is applied. A spark jumping the gap acts as an ignition source for gaseous fuels, and has been used in many installations where a standing pilot is impractical or is now illegal. Spark ignition systems have certain drawbacks. A spark ignition system tends to generate radio frequency interference because of the nature of spark ignition equipment, and the spark also generates an audible noise that is distracting and undesirable.
A third type of ignition source has been used to a limited degree, and is a hot surface ignitor arrangement. A hot surface ignitor can be a loop or coil of high resistance wire that is energized to cause the wire to glow. This type of element has a number of drawbacks. One of the drawbacks is the fragile nature of the wire and its mounting. Another drawback is its very short life.
Other types of hot surface ignitors have been under development for a number of years. Typically they are ceramic elements that have a U-shaped configuration, or a serpentine configuration, to provide a resistance element that will glow to incandescence when an appropriate voltage is applied. Typically, the voltage applied to ceramic type elements is line voltage. These elements are normally made of silicon carbide, and provide a substantial mass that can be brought to a glowing level of heat for ignition of gaseous fuels. The silicon carbide and similar types of ignitors have many of the deficiencies of the other hot surface ignitor elements. They tend to have a limited life and are also quite fragile.
In using any of the hot surface ignition devices, it is desirable to be able to determine whether the ignitor, in fact, has reached an ignition temperature thus indicating that it has not been broken or fractured. Early attempts to use hot surface ignitors have used current measuring circuitry that, in one way or another, measured the current flow to the hot surface ignitor. The measurement of current was then converted into an indication of whether or not the hot surface ignitor had electrical continuity. If electrical continuity existed, that indication along with the level of current flow could be used as a measure of whether the hot surface ignitor in fact was reaching an ignition temperature for the fuel being used. This type of circuit arrangement is very costly to implement, and therefore has in many cases limited the use of hot surface ignitors as an ignition source for gaseous fuels. It is quite obvious that this type of arrangement would not have the noise problems, either electrical or audible, and therefore might be more desirable than a spark ignition source for gaseous fuel ignition.
A typical Hot Surface Ignition Control system is manufactured and sold by Honeywell under the type number S89C. This type of system utilizes electronic controls for the energization of the hot surface ignitor and the subsequent opening of a fuel or gas valve to a burner in a furnace or similar appliance. Devices such as the Honeywell S89C typically used a fixed time interval of energization of a hot surface ignitor for the generation of sufficient heat in the hot surface ignitor, and then the fuel or gas valve was opened. Only after the gas valve was opened and an absence of flame was detected, did the system know that the ignitor was not functioning properly. At this point the system would automatically shut down.